The subject matter disclosed herein relates generally to imaging systems, and more particularly to reducing polarization in imaging detectors.
In medical imaging, ionizing radiation detectors (e.g., detectors configured for detection of X-rays and/or Gamma-rays) may be used, for example, for high flux photon counting and medical imaging, which may also be referred to as molecular imaging (MI). For example, ionizing radiation detectors may be used for one or more of spectral-computed tomography (Spectral-CT or Color-CT, which is capable of counting photons and measuring their energy), single photon emission computed tomography (SPECT) in combination with CT (SPECT-CT), positron emission tomography (PET) combined with CT (PET-CT), or magnetic resonance imaging (MRI) combined with CT (MRI-CT).
In ionizing radiation detectors (e.g., CdZnTe (CZT) detectors) for high flux photon counting, such as used for example in Spectral-CT or Color-CT, a polarization effect may be produced by the formation of positive space charge in a detector. The space charge may reduce the internal electrical field in the detector, which may degrade performance of the detector. Degradation may also result by the attracting, by the positive space charge, of electrons toward a cathode, thereby reducing the drift of the electrons that should occur under ideal operation toward the anode via the detector bias (or internal field). Under relatively high flux of ionizing radiation, such as X-rays or Gamma-rays, a strong positive space-charge may be formed, which may cause the internal electrical field in the detector to collapse, resulting in cessation of operation of the detector.
When a detector is irradiated by relatively high ionizing radiation flux, the formation of a positive space charge may be mainly created by two causes. First, a positive space charge may be created due to ionization of long lifetime deep level hole traps. Second, a positive space charge may be created due to low or reduced mobility of holes that are outside of a hole trap. For example, each ionizing photon absorbed in the detector may create an electron-cloud and a holes-cloud. Under irradiation of a relatively high flux of ionizing photon, many electrons and hole clouds are formed in the detector by a large amount of ionizing photons absorbed in the detector. Due to low mobility of the hole clouds, the clouds are not collected by the cathode and instead remain in the detector bulk after most of the electrons clouds have been collected by the anodes. The hole clouds left in the detector bulk create a large positive space charge in the detector, which reduces the electrical field in the detector and attracts electrons toward the cathode, thereby degrading detector performance. While the use of relatively thin detectors with blocking cathodes operated at high bias voltage may improve hole drift velocity and detector performance while maintaining relatively low leakage current, further improvement is still desired, and such relatively thin detectors may be unsuited for use with Nuclear Imaging and X-ray applications due to their relatively low stopping power.